Positron Emission Tomography (PET) is a functional imaging technique with the potential to quantify the rates of biological processes in vivo. The availability of short-lived positron-emitting isotopes of carbon, nitrogen, oxygen and especially fluorine allows virtually any compound of biological interest to be labeled in trace amounts and introduced into the body for imaging with PET. The distribution of the tracer is imaged dynamically, allowing the rates of biological processes to be calculated using appropriate mathematical models. PET imaging can provide diagnosis for symptoms of diseases such as cancer, Alzheimer's disease, head trauma, and stroke. It is clear that PET technology is playing a prominent and increasingly visible role in modern research and clinical diagnosis. However, to allow exploitation of the full potential of this promising technique, there is urgent need for both improvement in the performance of PET systems and reduction in their cost. Both of these factors are strongly influenced by the available detector technology. Scintillation crystals coupled to photomultiplier tubes are currently used as detectors in PET systems. Important requirements for the scintillation crystals used in PET systems include fast response, high sensitivity, high light output, good proportionality, high energy and timing resolution, and low cost. Traditional single crystals scintillators (such LSO, BGO and GSO) which are currently used in commercial PET scanners show considerable limitations either in performance aspects (such as low light output, poor proportionality and slow response) or in cost and availability aspects. With none of the well-established scintillators able to satisfy all the stated requirements of PET, the goal of the proposed effort is to investigate a new garnet scintillator that provides high gamma-ray stopping efficiency, high light yield, and fast decay time. The energy and timing resolution of these new garnetscintillators surpass those for the existing PET scintillators. Furthermore, due to their cubic structure and the associated physical and optical isotropy, these garnet scintillators can be fabricated using ceramic fabrication techniques, with properties rivaling those of the best crystals, yet remaining cost effective for fabrication in large quantities. This approach involves developing garnet detectors in the form of optical ceramics (OCs), rather than the single crystals. Consolidation of powder into a fully dense ceramic provides many advantages over traditional single crystal growth, such as lower fabrication temperatures and simpler processing equipment. It is the aim of this proposal to produce new garnet scintillators doped with Ce3+ and Pr3+ in the form of a fully transparent optical ceramics, which display scintillation performance better than that of best single crystals used currently in PET at significantly lower cost with wider availability. Construction and evaluation of detector modules for PET imaging based on optically transparent ceramic garnet scintillators is also planned in the proposed effort.